1. Field of the Invention.
The apparatus of the present invention relates to apparatus capable of being employed downhole in the drill string to sense borehole directional information, temperature, and formation evaluation parameters, and to convey the information to a surface receiver, without withdrawing the apparatus from the hole (referred to as Measurement While Drilling, or MWD, tools). The present invention relates to an improved means to conserve battery energy use by an MWD tool by limiting operation of the MWD tool to periods of desired drilling fluid flow conditions by a switch actuator responsive to a pressure differential generated by fluid flow, yet isolated from the drilling fluid and of an inherently self cleaning design, the switch actuator actuating a switch at the desired times. The invention further relates to a combined flow and rotation sensitive apparatus for activating an MWD tool. The present invention further relates to an improved flow restrictor pilot valve assembly comprising a dual dashpot for controlling valve stem movement, a volume compensating means, and inherently non-plugging and self-cleaning fluid passages.
2. Description of the Related Art.
Originally, oil and gas wells boreholes were not intentionally deviated but rather were drilled as vertical wells, with the drilling rig and surface location of the well situated directly over the desired reservoir penetration target. In particular with the development of the offshore oil and gas industry, directional wells have become quite commonplace. Directional wells are wells that are intentionally deviated from vertical in order to penetrate a subsurface target displaced horizontally some distance from the surface location. It is of critical importance to accurately survey the wellbore to know its angle (deviation from vertical) and azimuth (direction relative to a fixed direction). In offshore operations, drilling directional wells permits multiple wells to be drilled from a single offshore structure, with the surface location of each well displaced only a few feet from one another.
Directional wells in onshore situations are increasingly common. For example, the surface location for a vertical well may be in an environmentally sensitive area, and regulations might make drilling in such an area either prohibited or very expensive; a directional well may permit the surface location to be in an area not having significant environmental concerns.
Additional uses for onshore and offshore directional wells include more efficient exploitation of subsurface reservoirs, by drilling horizontal wells which penetrate multiple generally vertically disposed formation fractures, and wells that penetrate multiple subsurface reservoir targets displaced from one another.
Measurement While Drilling (MWD) tools permit taking multiple directional wellbore surveys without inserting additional survey tools downhole. MWD tools are incorporated into the drillstring downhole and accurately measure wellbore inclination and direction, toolface, temperature, along with other desired parameters, while drilling fluid is being circulated down the drillstring and back to the surface. In addition, the MWD tool, in certain embodiments, can measure various formation evaluation parameters, such as gamma radiation. The tool codes this information into a series of electrical signals which are sent to an electric solenoid or similar means which triggers operation of a flow restrictor pilot valve. The flow restrictor restricts drilling fluid flow in a controlled manner so as to send fluid pressure pulses to the surface for receipt and decoding into borehole directional information and other information as described above.
A downhole battery powers the MWD tool, and it is desirable to conserve battery energy to the greatest extent possible so as to prolong the downhole life of the MWD tool. Energy is conserved by activating the MWD only when desired; for directional survey information, only when drilling is not ongoing (and the drillstring and MWD tool are therefore not rotating, or are rotating only within desired parameters), and when drilling fluid is being circulated. The MWD tool should be "turned off", or in a dormant state, except when combined conditions of non- or slow rotation and sufficient desired circulation rate exist. Typically, the survey device package within the MWD tool has an internal rotation sensor which turns off the tool when the tool is rotating outside of desired parameters. For formation evaluation MWD tools, data may be measured and transmitted while rotary drilling is ongoing, although non-activation during periods of non-rotation is required. It is desirable, then, to additionally have a means responsive to drilling fluid circulation rate to turn the tool on and off.
Prior efforts to incorporate a flow sensitive means to turn the MWD on and off included a rotary turbine. Drilling fluid flow was routed past a shaft-mounted rotatable turbine or propeller which would spin in response. A sensing means responsive to rotation of the turbine shaft would then activate the MWD tool. The turbine means inherently has several operational difficulties. The rotating turbine and shaft assembly is subject to excessive mechanical wear. The abrasive and often corrosive nature of the drilling fluid tends to degrade all exposed turbine parts and invade the internal turbine mechanism unless a perfect seal is in effect about the turbine shaft. Further, the rotation of the turbine could be stopped by solids accumulating about the turbine blades and/or shaft.
The fluid isolated, flow-responsive switch actuator of the present invention avoids the problems presented by turbine means. The present invention utilizes a switch actuator responsive to a pressure differential caused by drilling fluid flow through an annular passage. Upstream and downstream bellows seal a fluid filled reservoir; contained within and cooperatively engaging the upstream bellows is a push-type switch. The downstream portion of the reservoir contains various operating parts of the flow restrictor pilot valve assembly, with the downstream bellows sealing around the stem of the flow restrictor pilot valve. The apparatus is disposed within the bore of a tubular mandrel in the drillstring, or simply disposed within the drillstring bore. Drilling fluid flows through the annulus between the apparatus and the bore, and the friction pressure loss along the longitude of the annulus results in a pressure differential between the two bellows. Ports expose both bellows to the mud flowstream. In addition, both upstream and downstream ports provide constant drilling fluid flow past the bellows to effect a self-cleaning design. As drilling fluid circulation commences, the pressure differential builds between the upstream and downstream points until a pre-set force is reached on the upstream bellows, and in turn on the push-switch therein, when the switch will be actuated and the MWD tool activated. Therefore, since directional surveys and formation evaluation readings are taken only while circulating, the MWD tool is in a dormant state and power consumption is minimized during periods in which the data measurement is not being done.
The resulting required tool length to yield an appropriate pressure differential between two points can cause problems in the assembling and handling of the pressure differential switch actuator apparatus. As a general rule, shorter downhole tools are preferred over longer downhole tools for increased durability and reduction of resonance and vibration problems. The apparatus of the present invention achieves the required spacing needed for adequate pressure differential, while not requiring excessive overall tool length, by utilizing the flow restrictor pilot valve length and internal volume as an integral part of the pressure differential flow switch actuator. By doing so, a greater utilization of existing tool volume is made; in effect, tool length once used solely for the flow restrictor pilot valve is now additionally used to provide pressure port spacing and thus yield a desired pressure differential.
Additionally, the present invention comprises several improvements to the flow restrictor pilot valve assembly. Valve stem travel is controlled at each end of the stem stroke, after release of the stem from latched positions at each end of the stroke, by a dual dashpot arrangement. A volume compensating means allows for volume changes in the switch actuator reservoir caused by the longitudinal movement of the valve stem shaft in the reservoir. Further, the downstream ports admitting drilling fluid to the downstream bellows and allowing fluid flow through the pilot valve comprise at least one aperture having a cross-section area larger at the interior wall of the apparatus than at the exterior wall. As a result, solids in the drilling fluid flowstream larger than the minimum aperture opening can neither pass through the aperture nor bridge and plug the opening. Interior clearances of the parts of the pilot valve are sized correspondingly so as to prevent clogging by entrained solids, the combination of the aperture size and internal clearances creating a non-plugging pilot valve assembly.
The flow switch pilot valve assembly above described represents significant improvement over prior apparatus such as U.S. Pat. No. 5,103,430 to Jeter, et al. discloses a mud pulse signal generator. However, the Jeter apparatus employs only a single dashpot assembly for dampening and control of valve stem movement. The dual dashpot of the present invention provides desired temporal control of the pilot valve stem in both directions of stem stroke, and further prevents "bounce back" of the valve stem off of the servo passage seat and the resulting potentially erroneous mud pulse triggers. In addition, the present invention incorporates an inherently self-cleaning and non-plugging design of the pilot valve, with the combination of the unique downstream slot cross sectional area and the internal fluid clearances. Integration of the pilot valve mechanism into the pressure actuated switch actuator, and incorporation of a volume compensating means to provide for valve stem movement within the actuator reservoir, represent significant advances over prior apparatus.
It is an object of the present invention to provide an improved apparatus for conserving the energy of an MWD tool battery by activating the MWD device only during combined desired conditions of drilling fluid flow rate and MWD tool rotation. Another object of the present invention is to provide a flow rate sensitive switch actuator responsive to a pressure differential between two points, created by drilling fluid flow along a longitude of the MWD tool. Yet another object is to provide a pressure differential operated switch actuator that is self cleaning due to fluid flow past the actuator, and wherein the switch is fluid isolated from the potentially abrasive and/or corrosive drilling fluid.
Another object is to provide a switch actuator and flow restrictor pilot valve assembly that dampens and temporally controls movement of the pilot valve stem at each end of the stem stroke, that provides volume compensation in the switch actuator reservoir so that valve shaft movement does not interfere with the switch actuator function, and has downstream fluid and pressure ports and internal fluid passage sizing so as to be inherently non-plugging.
Still another object is to integrate the MWD tool pilot valve length into the pressure differential switch actuator and thereby create a simple, short, reliable switch actuator by utilizing tool length and volume, formerly dedicated to a single purpose, for multiple purposes.